The present invention relates to a wastegated turbocharger. In particular, the invention relates to a dual wastegate port and valve assembly for a turbocharger provided with a divided inlet turbine.
Turbochargers are well known devices for supplying air to the intake of an internal combustion engine at pressures above atmospheric pressure (boost pressures). A conventional turbocharger essentially comprises an exhaust gas driven turbine wheel mounted on a rotatable shaft within a turbine housing connected downstream of an engine outlet manifold. Rotation of the turbine wheel rotates a compressor wheel mounted on the other end of the shaft within a compressor housing. The compressor wheel delivers compressed air to the engine intake manifold. The turbocharger shaft is conventionally supported by journal and thrust bearings, including appropriate lubricating systems, located within a central bearing housing connected between the turbine and compressor wheel housings.
In known turbochargers, the turbine stage comprises a turbine chamber within which the turbine wheel is mounted. An annular inlet passageway is defined between facing radial walls arranged around the turbine chamber. An inlet volute is arranged around the inlet passageway and an outlet passageway extends from the turbine chamber. The passageways and chambers communicate such that pressurised exhaust gas admitted to the inlet chamber flows through the inlet passageway to the outlet passageway via the turbine and rotates the turbine wheel. It is also known to improve turbine performance by providing vanes, referred to as nozzle vanes, in the inlet passageway so as to deflect gas flowing through the inlet passageway toward the direction of rotation of the turbine wheel.
Turbines may be of a fixed or variable geometry type. Variable geometry turbines differ from fixed geometry turbines in that the size of the inlet passageway can be varied to optimise gas flow velocities over a range of mass flow rates so that the power output of the turbine can be varied to suite varying engine demands. For instance, when the volume of exhaust gas being delivered to the turbine is relatively low, the velocity of the gas reaching the turbine wheel is maintained at a level which ensures efficient turbine operation by reducing the size of the annular inlet passageway. Turbochargers provided with a variable geometry turbine are referred to as variable geometry turbochargers.
A turbocharger turbine may be provided with a valve controlled bypass port referred to as a wastegate, to enable control of the turbocharger boost pressure and/or shaft speed. A wastegate valve (typically a poppet type valve) is controlled to open the wastegate port when the pressure of the boost air increases toward a pre-determined level, thus allowing some of the exhaust gas to bypass the turbine wheel. Typically the wastegate port opens into a bypass passageway which diverts the bypass gas flow to the turbine outlet or vents it to atmosphere. The wastegate valve may be actuated by a variety of means, including electric actuators, but is more typically actuated by a pneumatic actuator operated by boost pressure delivered by the compressor wheel.
Turbochargers intended for installation on an engine with two banks of cylinders may be provided with a turbine which has a divided inlet volute. This provides separate parallel gas inlet flow paths to the turbine wheel from each of the engine cylinder banks so that separate impulses of the exhaust gases will be more effectively transmitted to the turbine wheel. Wastegating of such a divided inlet turbine requires two wastegate ports, one for each separate inlet path, controlled by a dual wastegate valve mechanism.
The provision of a dual wastegate valve mechanism which provides simultaneous opening and closing of dual wastegate ports whilst ensuring each valve seats correctly in a closed position is problematic. Early examples of dual wastegate valve mechanism comprising a pair of valve members mounted to a single support arm is disclosed in Japanese Patent Application laid-open publication no. 57-137619. This discloses a dual wastegate valve mechanism comprising two valve members secured directly to a linking support member so that movement of the support member moves both valve members together. However, even a relatively slight distortion of the support member can result in improper closing of one or both valves so that exhaust gas flow can leak through the respective wastegate port. An improvement on this mechanism is disclosed in Japanese utility model application laid-open publication no. 62-183033. In this mechanism, each valve member is loosely coupled to the support member by a pin which allows some movement of the valve member relative to the support member. This freedom of movement accommodates minor distortion in the support member. However, excessive freedom of movement between each valve member and the support member can result in premature opening of one or other valve member before the boost pressure reaches the predetermined level.
A dual wastegate valve mechanism proposed to further improve upon the mechanisms described above is disclosed in U.S. Pat. No. 5,046,317. In this mechanism, a pair of wastegate valve members (poppets) are loosely mounted to a linking support member and have limited freedom of movement relative to the support member. In addition, the support member is pivotally connected to a support arm at a mid point between the two valve members to allow a small degree of pivoting movement of the support member itself relative to the wastegate ports. The combined freedom of movement of the valve members on the support member, and the support member on the support arm is said to accommodate differences in the valve seat height whilst ensuring each valve member firmly sits properly on its valve seat when the wastegate ports are closed.
Another dual wastegate valve mechanism is disclosed in U.S. Pat. No. 5,996,348. This takes a different approach to the provision of a mechanism which allows the simultaneous opening and closing of the two valve members whilst permitting a limited freedom of movement between the two valve members. Each valve member (poppet) is mounted to a respective arm extending at an angle from a respective shaft so that rotation of the shaft raises or lowers the respective poppet to open and close the wastegate port. The two shafts are concentrically arranged and are welded together at their ends remote from the poppet valves, at which point they are connected to a common actuation lever, such that movement of the lever rotates both shafts together, to simultaneously open or close the wastegate ports. Although the shafts are welded together at one end, they are torsionally independent which is intended to allow the shafts to separately accommodate tolerance build up or deformation of the valve seats to enable each poppet to seat correctly when closed.
Whilst the more recent dual wastegate valve mechanisms proposed in U.S. Pat. Nos. 5,046,317 and 5,996,348 address shortcomings in earlier proposals, they are relatively complicated mechanisms and are prone to wear.
It is an object of the present invention to provide an improved dual wastegate valve mechanism that provides opening and closing of dual wastegate ports.